CMOS circuits exhibit two temperature dependence regions as a function of the operating voltage. These two temperature dependence regions are Normal Temperature Dependence (NTD) region and Reverse Temperature Dependence (RTD) region. In the NTD region, transistor drive current decreases with increasing temperature. In the RTD region, transistor drive current increases with higher temperatures.
Generally, NTD effects are observed at high operating voltages and RTD effects are observed at low operating voltages, with a temperature insensitive supply voltage (Vins) separating these two regions. FIG. 1 is a plot 100 showing NTD 102 and RTD 103 regions for a processor with Vins 101 separating the two regions. The x-axis of the plot 100 is processor frequency while the y-axis of the plot 100 is supply voltage to the processor. When the processor operates at Vins 101, the transistor drive current in the processor is insensitive to temperature changes.
Effects of RTD are becoming more pronounced with transistor scaling to smaller geometries. For example, High-K/Metal Gate (HK/MG) transistor devices exhibit a higher Vins (and therefore a more pronounced RTD effect) due to stronger threshold voltage temperature dependence, compared to transistor devices using the poly-silicon gate and silicon oxide technology. In addition, since most processors operate at lower voltages (close to VCCmin, which is the lowest operating voltage level) to save power, the processors will operate most of the time in the RTD region. Since speed of transistors in a processor gets slower at lower temperatures in the RTD region, data paths in the processor may violate timing specifications causing the processor operations to become unreliable.